Test strips are often used for qualitative and quantitative analysis of blood components. The test strips are sometimes constructed so that the sample application area and the detection area are stacked above one another in a vertical axis. However, this type of construction is associated with a number of problems. For example, when the test strip is inserted into an instrument for measurement, the potentially infectious sample material may contact parts of the optical reader and result in contamination. Thus, spatial separation between the sample application area and detection zone is often desired, i.e., lateral flow strips. Most conventional lateral flow strips are designed for test samples that are readily available in large quantities (e.g., urine). However, when the test sample is blood, the collection of a large sample may cause undue pain to the patient. Thus, one technique that has been utilized to accommodate smaller test sample volumes is to “spot” the sample directly on the membrane surface. Thereafter, a diluent is used to wash away the test sample and carry it to the detection zone. Unfortunately, variations associated with sample transfer and diffusion of the sample to the membrane result in a flow that is largely uncontrolled and uneven before reaching the detection zone. This may have an adverse effect on the accuracy of the device because the amount of analyte and/or label captured across the detection zone is not consistent at the time of measurement.
In addition, various tests on blood samples require separation of the red blood cell components from the sample to obtain plasma or serum that is essentially free of red blood cells. The sample can then be used in various assays without interference from red blood cell components. In this regard, filter arrangements have been proposed for production of serum or plasma from whole blood. For example, U.S. Pat. No. 5,423,989 describes a membrane filtering arrangement with a first coarse membrane coated with a fibrous protein and a second fine membrane for removing red blood cells from a test sample. The blood sample is generally one or several small drops having a volume of from 10 to 100 microliters. The filtering membranes are arranged in a holder above a sample receiving element, and the blood sample is introduced into a well above the holder so that the blood flows through the coarse membrane and fine membrane before being deposited onto the sample receiving element, which may be round, square, rectangular, quadrilateral, or polygonal. The reference describes that the sample receiving element may be used in any convenient manner, including being diluted in an assay medium.
The arrangement according to the '989 patent is relatively complicated in that it calls for support structure for the filtering membranes and sample receiving element, and requires the user to separate the sample element from the structure for further analysis. The sample element may not be a size or configuration for direct use in an assay device, and may need to be further reconfigured (e.g., cut) for operation with the assay device.
As such, a need currently exists for a simple and efficient technique for metering and filtering a low volume blood test sample such that a known volume of blood plasma or serum may be easily transferred to a detection zone of a lateral flow assay device.